Why not leave the corals in your 50 gal on an egg crate rack, and get a cheep 20 gal to treat your fish. You'll need it later anyway for future quarantines and treatments.
Fresh water dips are risky and are not known to have any effect on cryptocaryon, they are burrowed into the skin and the slime on the fish protects them.
This is the method I always use for hyposalinity treatments. Be most carefully as you get down to the lower specific gravity. Almost all fish with respond to hypo salinity with increased vigour and a ravenous appetite. If the fish stop eating or show distress raise the salinity a bit. It is possible to kill fish with any treatment, that is why a lot of people just live with cryptocaryon.